The party was founded in 1951, when the Indochinese Communist Party (ICP) was divided into separate Cambodian, Lao, and Vietnamese communist parties. The decision to form a separate Cambodian communist party had been taken at the ICP congress in February the same year. Different sources claim different dates for the exact founding and the first congress of the party. The party congress did not elect a full Central Committee, but instead appointed a 'Party Propagation and Formation Committee'.[2] At the time of its formation, the Cambodian party was called Khmer People's Revolutionary Party. The Indochinese Communist Party had been heavily dominated by Vietnamese, and the KPRP was actively supported by the Vietnamese party during its initial phase of existence. Due to the reliance on Vietnamese support in the joint struggle against French colonial rule, the history of the party would later be rewritten, stating 1960 as the year of foundation of the party.[3]

According to Democratic Kampuchea's version of party history, the Viet Minh's failure to negotiate a political role for the KPRP at the 1954 Geneva Conference represented a betrayal of the Cambodian movement, which still controlled large areas of the countryside and which commanded at least 5,000 armed men. Following the conference, about 1,000 members of the KPRP, including Son Ngoc Minh, made a "Long March" into North Vietnam, where they remained in exile. In late 1954, those who stayed in Cambodia founded a legal political party, the Krom Pracheachon, which participated in the 1955 and the 1958 National Assembly elections. In the September 1955 election, it won about 4% of the vote but did not secure a seat in the legislature. Members of the Pracheachon were subject to constant harassment and to arrests because the party remained outside Sihanouk's Sangkum. Government attacks prevented it from participating in the 1962 election and drove it underground. It is speculated that the decision of Pracheachon to file candidates for the election had not been approved by the WPK.[3] Sihanouk habitually labeled local leftists the Khmer Rouge, a term that later came to signify the party and the state headed by Pol Pot, Nuon Chea, Ieng Sary, Khieu Samphan, and their associates.

During the mid-1950s, two KPRP factions, the "urban committee" (headed by Tou Samouth), and the "rural committee" (headed by Sieu Heng), emerged. In very general terms, these groups espoused divergent revolutionary lines. The prevalent "urban" line, endorsed by North Vietnam, recognized that Sihanouk, by virtue of his success in winning independence from the French, was a genuine national leader whose neutralism and deep distrust of the United States made him a valuable asset in Hanoi's struggle to "liberate" South Vietnam. Champions of this line hoped that the prince could be persuaded to distance himself from the right wing and to adopt leftist policies. The other line, supported for the most part by rural cadres who were familiar with the harsh realities of the countryside, advocated an immediate struggle to overthrow the "feudalist" Sihanouk. In 1959 Sieu Heng defected to the government and provided the security forces with information that enabled them to destroy as much as 90% of the party's rural apparatus. Although communist networks in Phnom Penh and in other towns under Tou Samouth's jurisdiction fared better, only a few hundred communists remained active in the country by 1960.

During the 1950s, Khmer students in Paris organized their own communist movement, which had little, if any, connection to the hard-pressed party in their homeland. From their ranks came the men and women who returned home and took command of the party apparatus during the 1960s, led an effective insurgency against Sihanouk and Lon Nol from 1968 until 1975, and established the regime of Democratic Kampuchea.

Pol Pot, who rose to the leadership of the communist movement in the 1960s, was born in 1928 (some sources say in 1925) in Kampong Thum Province, northeast of Phnom Penh. He attended a technical high school in the capital and then went to Paris in 1949 to study radio electronics (other sources say he attended a school for printers and typesetters and also studied civil engineering).

Another member of the Paris student group was Ieng Sary. He was a Chinese-Khmer born in 1930 in South Vietnam. He attended the elite Lycée Sisowath in Phnom Penh before beginning courses in commerce and politics at the Institut d'Etudes Politiques de Paris (more widely known as Sciences Po) in France. Khieu Samphan, considered "one of the most brilliant intellects of his generation," was born in 1931 and specialized in economics and politics during his time in Paris. In talent he was rivaled by Hou Yuon, born in 1930, who studied economics and law. Son Sen, born in 1930, studied education and literature; Hu Nim, born in 1932, studied law.

Most members of the Paris student group came from landowner or civil servant families. Three of the Paris group forged a bond that survived years of revolutionary struggle and intraparty strife, Pol Pot and Ieng Sary married Khieu Ponnary and Khieu Thirith (also known as Ieng Thirith), purportedly relatives of Khieu Samphan. These two well-educated women also played a central role in the regime of Democratic Kampuchea.

At some time between 1949 and 1951, Pol Pot and Ieng Sary joined the French Communist Party. In 1951 the two men went to East Berlin to participate in a youth festival. This experience is considered to have been a turning point in their ideological development. Meeting with Khmers who were fighting with the Viet Minh (and whom they subsequently judged to be too subservient to the Vietnamese), they became convinced that only a tightly disciplined party organization and a readiness for armed struggle could achieve revolution. They transformed the Khmer Students' Association (KSA), to which most of the 200 or so Khmer students in Paris belonged, into an organization for nationalist and leftist ideas. Inside the KSA and its successor organizations was a secret organization known as the Cercle Marxiste. The organization was composed of cells of three to six members with most members knowing nothing about the overall structure of the organization. In 1952 Pol Pot, Hou Yuon, Ieng Sary, and other leftists gained notoriety by sending an open letter to Sihanouk calling him the "strangler of infant democracy." A year later, the French authorities closed down the KSA. In 1956, however, Hou Yuon and Khieu Samphan helped to establish a new group, the Khmer Students' Union. Inside, the group was still run by the Cercle Marxiste.

The doctoral dissertations written by Hou Yuon and Khieu Samphan express basic themes that were later to become the cornerstones of the policy adopted by Democratic Kampuchea. The central role of the peasants in national development was espoused by Hou Yuon in his 1955 thesis, The Cambodian Peasants and Their Prospects for Modernization, which challenged the conventional view that urbanization and industrialization are necessary precursors of development. The major argument in Khieu Samphan's 1959 thesis, Cambodia's Economy and Industrial Development, was that the country had to become self-reliant and end its economic dependency on the developed world. In its general contours, Khieu's work reflected the influence of a branch of the "dependency theory" school, which blamed lack of development in the Third World on the economic domination of the industrialized nations.

After returning to Cambodia in 1953, Pol Pot threw himself into party work. At first he went to join with forces allied to the Viet Minh operating in the rural areas of Kampong Cham Province (Kompong Cham). After the end of the war, he moved to Phnom Penh under Tou Samouth's "urban committee" where he became an important point of contact between above-ground parties of the left and the underground secret communist movement. His comrades, Ieng Sary and Hou Yuon, became teachers at a new private high school, the Lycée Kambuboth, which Hou Yuon helped to establish. Khieu Samphan returned from Paris in 1959, taught as a member of the law faculty of the University of Phnom Penh, and started a left-wing, French-language publication, L'Observateur. The paper soon acquired a reputation in Phnom Penh's small academic circle. The following year, the government closed the paper, and Sihanouk's police publicly humiliated Khieu by beating, undressing and photographing him in public—as Shawcross notes, "not the sort of humiliation that men forgive or forget." Yet the experience did not prevent Khieu from advocating cooperation with Sihanouk in order to promote a united front against United States activities in South Vietnam. As mentioned, Khieu Samphan, Hou Yuon, and Hu Nim were forced to "work through the system" by joining the Sangkum and by accepting posts in the prince's government.

On September 28-September 30, 1960, twenty-one leaders of the KPRP held a secret congress in a vacant room of the Phnom Penh railroad station. It is estimated that 14 delegates represented the 'rural' faction and seven the 'urban' faction.[4] This pivotal event remains shrouded in mystery because its outcome has become an object of contention (and considerable historical rewriting) between pro-Vietnamese and anti-Vietnamese Khmer communist factions. At the meeting the party was renamed as the Workers Party of Kampuchea (WPK). The question of cooperation with, or resistance to, Sihanouk was thoroughly discussed. A new party structure was adopted. For the first time since, a permanent Central Committee was appointed with, Tou Samouth, who advocated a policy of cooperation, as the general secretary of the party. His ally, Nuon Chea (also known as Long Reth), became deputy general secretary; however, Pol Pot and Ieng Sary were named to the Central Committee to occupy the third and the fifth highest positions in the party hierarchy. Another committee member was veteran communist Keo Meas. In Democratic Kampuchea, this meeting would later be projected as the founding date of the party, consciously downplaying the history of the party prior to Pol Pot's ascent to leadership.[3]

On July 20, 1962, Tou Samouth was murdered by the Cambodian government. In February 1963, at the WPK's second congress, Pol Pot was chosen to succeed Tou Samouth as the party's general secretary. Tou's allies, Nuon Chea and Keo Meas, were removed from the Central Committee and replaced by Son Sen and Vorn Vet. From then on, Pol Pot and loyal comrades from his Paris student days controlled the party center, edging out older veterans whom they considered excessively pro-Vietnamese.

In July 1963, Pol Pot and most of the central committee left Phnom Penh to establish an insurgent base in Ratanakiri Province in the northeast. Pol Pot had shortly before been put on a list of thirty-four leftists who were summoned by Sihanouk to join the government and sign statements saying Sihanouk was the only possible leader for the country. Pol Pot and Chou Chet were the only people on the list who escaped. All the others agreed to cooperate with the government and were afterward under 24-hour watch by the police.

The region Pol Pot and the others moved to was inhabited by tribal minorities, the Khmer Loeu, whose rough treatment (including resettlement and forced assimilation) at the hands of the central government made them willing recruits for a guerrilla struggle. In 1965, Pol Pot made a visit of several months to North Vietnam and China. He probably received some training in China, which must have enhanced his prestige when he returned to the WPK's liberated areas. Despite friendly relations between Sihanouk and the Chinese, the latter kept Pol Pot's visit a secret from Sihanouk. In 1971, the party changed its name to the "Communist Party of Kampuchea" (CPK).[6] The change in the name of the party was a closely guarded secret. Lower ranking members of the party and even the Vietnamese were not told of it and neither was the membership until many years later. The party leadership endorsed armed struggle against the government, then led by PrinceNorodom Sihanouk. In 1967, several small-scale attempts at insurgency were made by the CPK but they met with little success.

In 1968, the Khmer Rouge launched a national insurgency across Cambodia. Though North Vietnam had not been informed of the decision, its forces provided shelter and weapons to the Khmer Rouge after the insurgency started. The guerrilla forces of the party were baptized as the Revolutionary Army of Kampuchea. Vietnamese support for the insurgency made it impossible for the ineffective and poorly-motivated Royal Cambodian Army to effectively counter it.

The political appeal of the Khmer Rouge was increased as a result of the situation created by the removal of Sihanouk as head of state in 1970. Premier Lon Nol, with the support of the National Assembly, deposed Sihanouk. Sihanouk, in exile in Beijing, made an alliance with the Kampuchean Communist Party and became the nominal head of a Khmer Rouge-dominated government-in-exile (known by its French acronym, GRUNK) backed by the People's Republic of China. Sihanouk's popular support in rural Cambodia allowed the Khmer Rouge to extend its power and influence to the point that by 1973 it exercised de facto control over the majority of Cambodian territory, although only a minority of its population.

The relationship between the massive carpet bombing of Cambodia by the United States and the growth of the Khmer Rouge, in terms of recruitment and popular support, has been a matter of interest to historians. Some historians have cited the U.S. intervention and bombing campaign (spanning 1965–1973) as a significant factor leading to increased support of the Khmer Rouge among the Cambodian peasantry. However, Pol Pot biographer David Chandler argues that the bombing "had the effect the Americans wanted – it broke the Communist encirclement of Phnom Penh".[7]Peter Rodman and Michael Lind claimed that the US intervention saved Cambodia from collapse in 1970 and 1973.[8][9] Craig Etcheson agreed that it was "untenable" to assert that US intervention caused the Khmer Rouge victory while acknowledging that it may have played a small role in boosting recruitment for the insurgents.[10]William Shawcross, however, wrote that the US bombing and ground incursion plunged Cambodia into the chaos Sihanouk had worked for years to avoid.[11]

The Vietnamese intervention in Cambodia, launched at the request of the Khmer Rouge,[12] has also been cited as a major factor in their eventual victory, including by Shawcross.[13] Vietnam later admitted that it played "a decisive role" in their seizure of power.[14] China "armed and trained" the Khmer Rouge during the civil war and continued to aid them years afterward.[15]

The leadership of the Khmer Rouge was largely unchanged between the 1960s and the mid-1990s. The Khmer Rouge leaders were mostly from middle-class families and had been educated at French universities.

The Standing Committee of the Khmer Rouge's Central Committee ("Party Center") during its period of power consisted of:

In power, the Khmer Rouge carried out a radical program that included isolating the country from foreign influence, closing schools, hospitals and factories, abolishing banking, finance and currency, outlawing all religions, confiscating all private property and relocating people from urban areas to collective farms where forced labor was widespread. The purpose of this policy was to turn professional and urban Cambodians, or "Old People", into "New People" through agricultural labor. These actions resulted in massive deaths through executions, work exhaustion, illness, and starvation.

In Phnom Penh and other cities, the Khmer Rouge told residents that they would be moved only about "two or three kilometers" outside the city and would return in "two or three days." Some witnesses say they were told that the evacuation was because of the "threat of American bombing" and that they did not have to lock their houses since the Khmer Rouge would "take care of everything" until they returned. These were not the first evacuations of civilian populations by the Khmer Rouge. Similar evacuations of populations without possessions had been occurring on a smaller scale since the early 1970s.

The Khmer Rouge attempted to turn Cambodia into a classless society by depopulating cities and forcing the urban population into agricultural communes through brutal totalitarian methods. The entire population was forced to become farmers in labour camps. During their four years in power, the Khmer Rouge overworked and starved the population, at the same time executing selected groups who had the potential to undermine the new state (including intellectuals) and killing many others for even minor breaches of rules.

Through the 1970s, and especially after mid-1975, the party was also shaken by factional struggles. There were even armed attempts to topple Pol Pot. The resultant purges reached a crest in 1977 and 1978 when thousands, including some important CPK leaders, were executed. The older generation of communists, suspected of having links with or sympathies for Vietnam, were targeted by the Pol Pot leadership.

For roughly two years after the CPK took power, it referred to itself as the "Angkar" (Khmer: អង្គការ; pronounced ahngkah; meaning 'The Organization'). However, on September 29, 1977, Pol Pot publicly declared the existence of the CPK in a five-hour-long speech.[2] He revealed the true character of the supreme authority in Cambodia, an obscure ruling body that had been kept in seclusion.

The CPK had been extremely secretive throughout its existence. Before 1975 the secrecy was needed for the party's survival and Pol Pot and his closest associates had relied on continuing the extreme secrecy in order to consolidate their position against those they perceived as internal enemies during their first two years of power. The revelation of the CPK's existence shortly before Pol Pot was due to travel to Peking resulted from pressure from China on the Khmer Rouge leaders to acknowledge their true political identity at a time that they increasingly depended on China's assistance against the threats from Vietnam. Accordingly, Pol Pot in his speech claimed that the CPK's foundation had been in 1960 and emphasized its separate identity from Vietnamese communism.[16] This secrecy continued even after the CPK took power. Unlike most Communist leaders, Pol Pot was never the object of a personality cult. It would be almost a year before it was confirmed that he was Saloth Sar, the man long cited as the CPK's general secretary.

By December 1978, because of several years of border conflict and the flood of refugees fleeing Cambodia, relations between Cambodia and Vietnam deteriorated. Pol Pot, fearing a Vietnamese attack, ordered a pre-emptive invasion of Vietnam. His Cambodian forces crossed the border and looted nearby villages. Despite American and Chinese aid, these Cambodian forces were repulsed by the Vietnamese.

In early 1979, a pro-Vietnamese group of CPK dissidents led by Pen Sovan held a congress (which they saw as the '3rd party congress', thus not recognizing the 1963, 1975 and 1978 party congresses as legitimate) near the Vietnamese border. Along with Heng Samrin, Pen Sovan was one of the foremost founding members of the Kampuchean United Front for National Salvation (KUFNS or FUNSK), after becoming disillusioned with the Khmer Rouge.[17] Effectively the CPK was then divided into two, with the Pen Sovan-led group constituting a separate party, the Kampuchean People's Revolutionary Party (now the Cambodian People's Party).[2]

^Dmitry Mosyakov, “The Khmer Rouge and the Vietnamese Communists: A History of Their Relations as Told in the Soviet Archives,” in Susan E. Cook, ed., Genocide in Cambodia and Rwanda (Yale Genocide Studies Program Monograph Series No. 1, 2004), p54ff. Available online at: www.yale.edu/gsp/publications/Mosyakov.doc "In April–May 1970, many North Vietnamese forces entered Cambodia in response to the call for help addressed to Vietnam not by Pol Pot, but by his deputy Nuon Chea. Nguyen Co Thach recalls: "Nuon Chea has asked for help and we have liberated five provinces of Cambodia in ten days.""